1. Technical Field of the Invention
The present invention relates to a resin composition, a melt-extruded article, an oriented article and a packaging material for foods obtained from these articles and furthermore relates to a process for producing such resin compositions. More particularly, the present invention relates to a resin composition including a polyvinyl alcohol resin (herein after abbreviated as "PVA") and specific compounds, and furthermore to a melt-extruded article, an oriented article, a film article and a food packaging film obtained from this resin composition, which have an excellent selectivity in gas permeability.
2. Description of the Prior Art
The food packaging industry requires packaging materials for various kinds of foods. For example, cheese products such as processed cheese, which are a type of fermented foodstuffs, have been conventionally coated with wax. Also plastic materials have been used to improve the appearance and to preserve the eatable portion.
Nowadays, there is a need for packaging materials for natural cheese. The natural cheese contains live bacteria to permit the ripening, and these bacteria generate carbon dioxide not only during the ripening step, but throughout the later stages such as distribution. Accordingly, if an ordinary gas impermeable plastic bag is used in packaging such cheese, the packaged bag will swell up with generated carbon dioxide and the bag is likely to break. Further the quality of the cheese will be deteriorated.
On the other hand, since oxygen accelerates the growth of molds and oxidizes the fat content of the cheese, packaging materials less permeable to oxygen gas have been desired for cheese packaging.
Packaging materials that are readily permeable to carbon dioxide, but less permeable to oxygen have been desired for preserving not only fermented foodstuffs such as cheese and miso, but also coffee beans. Generally, however, plastic packaging materials having higher gas transmission rate (hereinafter abbreviated as "GTR") to carbon dioxide also, almost without any exception, have a higher GTR to oxygen. Thus a need exists for a packaging material that has a low oxygen GTR and a high ratio of carbon dioxide GTR to oxygen GTR (hereinafter abbreviated as CO2/O2 GTR ratio).
U.S. Pat. No. 4,073,733 describes a process for producing a water permeable membrane by coagulating polyethylene glyco-containing PVA in a liquid phase. The resulting membrane is highly permeable to substances of intermediate molecular weight such as urea and vitamin B12 in a liquid phase. The products of this patent are not described as useful for food packaging materials for which selective gas permeability is required.
European published patent application 524,823 describes a resin composition containing PVA, polyethylene glycol (hereinafter abbreviated as "PEG") and a saponified copolymer of an ethylene and a vinyl acetate (hereinafter abbreviated as "EVOH") . Although a certain level of selective gas permeable film for food packaging was attained by a film formed from the resin composition of this disclosure, film-forming properties were poor due to the hardness of the resin composition itself.
Prior to the present invention the art has not provided packaging materials for fermented foodstuffs such as cheese products, and for coffee beans, having a higher carbon dioxide GTR and a lower oxygen GTR, and furthermore resin compositions for packaging material having excellent formability in the film producing process have also been required.